The proposed work includes: 1) the validation of a radioimmunoassay for serum pepsinogen II, the determination of the distribution of serum pepsinogen II levels in patients with peptic ulcer disease, gastritis and other disorders, and the study of the relationships between serum pepsinogen I and pepsinogen II levels, 2) the purification and characterization of the different isoforms of pepsinogens I and II, and 3) immunological studies of pepsins I and II.